Within recent years, the demand for mobile and other radio frequency (RF) communications has increased dramatically. To service this demand, the need for effective antennas to broadcast the RF signals has also increased.
While antennas come in many forms, one of the most widely used antennas, especially for mobile communications, is the half-wave dipole. A brief description of these half-wave dipole antennas will be useful.
As shown in FIG. 1, a half-wave dipole 100 is a two pole antenna with two symmetrical equal length legs 102, 104. Each of the legs 102 and 104 is a quarter wavelength (λ/4) long, so that the entire length of the antenna is a half wavelength (λ/2).
A half-wave dipole forms a known and predictable radiation pattern as shown in FIG. 2A. The radiation pattern shown in FIG. 2A is shaped somewhat like a doughnut and radiates generally in all directions. FIG. 2B shows a two-dimensional cross sectional view of the radiation pattern in FIG. 2A, where such view is commonly used to evaluate antenna radiation patterns.
The half-wave dipole is generally preferred to other dipole lengths, (e.g., λ/8, λ/4, etc.), because of its superior radiation pattern. Further, it is the shortest resonant wave antenna and it includes a radiation resistance of 73 Ohm, which is near the 75 Ohm characteristic impedance of commonly used transmission lines, thereby simplifying impedance matching.
The wavelength of a signal produced by a half-wave dipole is generally described by the equation:       λ    =          c      f        ,where λ is wavelength, c is the speed of light 3×108 m/s, and f is frequency. Hence, for a particular frequency, there is a known wavelength. Therefore the length (λ/2) of the half-wave dipole is generally dictated by the frequency to be transmitted. For example, a dipole to function at 6 GHz is will have a length of 25 mm (λ=50 mm), but a dipole that is to function at 3 GHz, will require a length of 50 mm (λ=100 mm). If adjustments are made to the antenna size without adjusting the frequency transmitted (for instance in an attempt to increase the antenna gain), the result is typically a less desirable radiation pattern.
Controlling the energy radiated (gain) and directivity of the radiation pattern is important. Increasing the gain is generally desirable as it will allow a signal to be received at further distances. A sample illustration of the radiation pattern from a dipole with increasing gain is shown in FIG. 3, illustrating radiation patterns for gains of 6 dBd, 3 dBd, and 5 dBd. Controlling the direction of focus of the radiated energy (the directivity) is also important—some applications require the radiated energy to be focused in a single direction while others require the energy to be more dispersed. Frequently, alterations to these two characteristics (gain and directivity) go hand-in-hand, e.g., focusing the signal in a particular direction will tend to increase the gain in that direction as well. Since the size of the antenna is not generally adjustable, other solutions to control these characteristics have been devised.
One solution is to form an array of antennas, arranged and spaced so that the energy radiated from each collectively adds together in a preferred direction and thereby increases the overall gain over that of a single antenna. Nonetheless, because of the use of multiple antennas, the size of such an array will tend to be larger than a single antenna.
Another solution for increasing gain and directivity is to use a reflective sheet 106 as shown in FIG. 4. When using a reflective sheet, the energy in one direction is reflected back and added to the energy generated in the opposite direction, resulting in increased gain. Such an antenna is generally spaced a quarter wavelength (λ/4) or longer (up to 3λ/8) from the reflector surface so that the reflected wavefronts are in phase (the field at the reflective sheet experiences a 180 degree phase shift, which is added to the 180 degree phase shift the wave experiences traveling from and to the antenna).
While flat reflectors tend to enhance directivity by essentially blocking the energy in a 180 degree range, finer directivity control can be had with shaped reflectors, e.g., a parabolic dish. The shape of such reflectors aids in focusing the energy radiated in a desired pattern. While the parabolic dish offers good gain and directivity control, it tends to be physically quite large. For instance, at 2.4 GHz, a to obtain a 20 dBi gain, a 900 mm dish is used.
Another solution for control of gain and directivity is a dielectric lens, sometimes called a Luneberg lens. Such a dielectric lens is composed of a dielectric material and is placed a calculated distance measured in wavelengths in front of an antenna in its far field. The wavefront is shaped by the lens in accordance with physics similar to optical lens theory. Such lenses can be concave, dispersing energy, or convex, focusing energy. Nonetheless, these multi-element structures tend to be burdensome to construct as well as being large, so they are not commonly used.
In an attempt to create a small-scale antenna, a metal patch has been placed on top of a dielectric substrate. For example, at 2.4 GHz, a patch antenna on a ceramic dielectric can be as small as 22×22×4 mm. Nonetheless, these antennas are typically very inefficient and do not have desirable gain characteristics. Typically the gain of these antennas is −8 dBi.
Although numerous methodologies for controlling gain and directivity as described above exist, given the vast growth in radio frequency communication, improvements to these antennas are always desirable. Moreover, to meet the demand for smaller and smaller devices, any antenna that can maintain gain for a particular frequency yet be built in a smaller form factor is desirable.